Online Computer Terms Dictionary - P

priority inversion

<parallel> The state of a concurrent system where a high priority task is
waiting for a low priority task which is waiting for a medium priority task. The
system may become unstable and crash under these circumstances.

In an operating system that uses multiple tasks, each task (or context) may be
given a priority. These priorities help the scheduler decide which task to run
next. Consider tasks, L, M, and H, with priorities Low, Medium, and High. M is
running and H is blocked waiting for some resource that is held by L. So long as
any task with a priority higher than L is runable, it will prevent task L, and
thus task H, from running.

Priority inversion is generally considered either as a high-level design failure
or an implementation issue to be taken into account depending on who is talking.
Most operating systems have methods in place to prevent or take inversion into
account. Priority inheritance is one method.

The most public instance of priority inversion is the repeated 'fail-safe'
rebooting of the Mars Pathfinder. base station ('Sagan Memorial
Station').